This application applies to tools for repairing damaged internal threads in blind holes.
The following is a tabulation of some prior art that presently appears relevant:
U.S. PatentsPat. No.Issue DatePatentee5,281,059Jan. 25, 1994Stuckle4,661,028Apr. 1987Sanger4,090,808May 23, 1978Nannen
A satisfactory tool has not existed for the repair of cross-threaded and otherwise damaged threaded blind holes. Most commonly, such damage is caused by forcing a misaligned bolt, or forcing a mismatched thread.
To be effective, a thread repairing tool must be aligned with the existing undamaged threads. Repair attempted with a conventional tap can prove disastrous, either destroying or severely weakening the remaining threads. Tools of the prior art have not accomplished this end effectively, or are overly complicated, therefore expensive, and are not downwardly scalable to accommodate a variety of hole sizes.
The tool disclosed by Stuckle, in U.S. Pat. No. 5,281,059 is representative of prior devices. A reduced diameter threaded portion is inserted past the damaged thread entry to engage with the interior undamaged threads. As with his citing of Sanger in U.S. Pat. No. 4,661,028, he uses a forcing cone (23) to expand the tool into mating engagement with the internal undamaged threads. With that forcing cone (23), his tool cannot be downscaled much, to accommodate smaller hole sizes. It also requires extensive machining of its several components, substantially increasing cost. His finger grips (20) are particularly problematic for forming/machining.
Stuckle cites a spark plug hole as his primary example. Most spark plugs have a fine-threaded 14 mm OD (approximately 9/16 in). A coarse thread of the same size being much deeper would require that his threaded element be considerably smaller, exacerbating his scalability problem.